1. Technical Field
Embodiments are generally related to two staged refrigeration systems, and more particularly to two staged refrigeration systems used in but not limited to residential air conditioning systems, roof top units, residential and commercial air source heat pumps, and water source heat pumps.
2. Discussion of Prior Art
Two staged refrigeration systems are widely used in both residential and commercial buildings. Typical systems maintain single zone room temperatures by staging one or more compressors on and off in two stages. Two staged refrigeration systems operate so that a first group of compressors activate during low load conditions or lower room temperature bias of the set-point. At a high system load, high bias of the room temperature set point, or after the first group of compressors have been active for an extended time period, another group of compressors activate. Compressor groups deactivate in stages when the building load decreases. Typical two staged refrigeration systems are comprised of compressors, a thermostat, a condensing unit, and a supply air fan with ON and AUTO modes of operation. The fan ON mode allows the supply air fan to run continuously regardless of the compressor status. In AUTO mode, the fan status is controlled so that both the compressor and supply air fan turn on and off simultaneously. A circulation fan (also called an indoor fan) supplies air to the conditioned space.
Over the years, engineers have worked to improve two staged refrigeration systems. Particular inventive emphasis has focused on minimizing over-sizing during the equipment selection process. Yet because cooling loads and heating loads vary substantially over the operative spectrum, these measures did not solve the excessive indoor humidity and noise levels, high energy consumption rates, and the high cost of compressor replacements and other necessary repairs often associated with the refrigeration systems.
Further development of the variable capacity compressor resulted in a more energy efficient refrigeration system with greater indoor comfort control. In this system, the compressor could modulate the cooling capacity to within a range of 15 to 100 percent.
The above mentioned development in turn led to the creation of a system comprising both a variable capacity compressor and variable frequency drive. In this new system, the fan speed and compressor capacity could be modulated to maintain the zone temperature set point and discharge air temperatures, respectively. Due to high retrofit costs, however, this system is not implemented in the millions of already existing refrigeration systems. Noise levels are still high under partial load conditions as well. US patent application 20090255278 to Lifson and Taras (2005), proposes a single integrated modulation device that controls a condenser fan, compressor, and indoor fan or, alternatively, three separate modulation devices that each control one of the three previously stated devices (condenser fan, compressor, and indoor fan) at the same speed. Results from lab experiments conducted by the inventor of the proposed application suggests that modulating the condenser fan at the same speed as the compressor and indoor fan will result in a 20+ energy penalty under most operating conditions. The optimizer proposed in the following application also utilizes a single modulation device, however since it controls only the compressor and indoor fan, is more energy efficient than the device proposed by Lifson and Taras.